Modern motor vehicle air brake systems, and particularly such systems as are used on large trucks, provide greater braking capacity than systems provided in the past. Although this increased capacity provides better braking characteristics for heavily loaded vehicles, this increased capacity also increases the sensitivity of the brakes during stopping of a lightly loaded vehicle. This makes it more difficult for the operator to execute a smooth stop, particularly at lower speeds, with a lightly loaded vehicle.
Prior art brake systems have included air brake blend back proportioning valves (that is, valves which provide an outlet pressure proportionally reduced from the inlet pressure at lower braking pressures and which blend back or equalize inlet and outlet presures pressures higher braking pressures), including those shown in U.S. Pat. Nos. 3,428,071 and 3,492,052.
The present invention departs from the structure and mode of operation of prior art braking systems by providing a novel braking system which includes a novel blend back proportioning valve having a housing with an inlet port and an outlet port. A shuttle piston is slidably disposed in the housing, and an outlet diaphragm includes a radially outer portion secured to the housing and a radially inner portion secured to the shuttle piston. At low braking pressures, the diaphragm has a predetermined lateral cross-sectional area exposed to outlet pressure and acting against the shuttle piston so that the shuttle piston acts as a differential area piston to provide proportional but smaller increases in outlet port pressure with respect to inlet port pressure. At higher braking pressures, the lateral cross-sectional area of the outlet diaphragm exposed to outlet pressure and acting against the shuttle piston is reduced to change the proportioning ratio of the valve and equalize inlet and outlet pressures.